Streptococcus pneumoniae (the pneumococcus) is the most common causative pathogen of CAP across a range of severities and patient ages.[13]Rice LB. Antimicrobial resistance in gram-positive bacteria. Am J Med. 2006 Jun;119(suppl 1):S11-9.
http://www.ncbi.nlm.nih.gov/pubmed/16735146?tool=bestpractice.com
[14]Cillóniz C, Ewig S, Polverino E, et al. Microbial aetiology of community-acquired pneumonia and its relation to severity. Thorax. 2011 Apr;66(4):340-6.
http://thorax.bmj.com/content/66/4/340.long
http://www.ncbi.nlm.nih.gov/pubmed/21257985?tool=bestpractice.com
[15]Cillóniz C, Ewig S, Polverino E, et al. Community-acquired pneumonia in outpatients: aetiology and outcomes. Eur Respir J. 2012 Oct;40(4):931-8.
http://erj.ersjournals.com/content/40/4/931.long
http://www.ncbi.nlm.nih.gov/pubmed/22267760?tool=bestpractice.com
[16]Cilloniz C, Torres A, Polverino E, et al. Community-acquired lung respiratory infections in HIV-infected patients: microbial aetiology and outcome. Eur Respir J. 2014 Jun;43(6):1698-708.
http://erj.ersjournals.com/content/43/6/1698.long
http://www.ncbi.nlm.nih.gov/pubmed/24525448?tool=bestpractice.com
[17]Almirall J, Boixeda R, Bolíbar I, et al. Differences in the etiology of community-acquired pneumonia according to site of care: a population-based study. Respir Med. 2007 Oct;101(10):2168-75.
http://www.resmedjournal.com/article/S0954-6111%2807%2900194-1/fulltext
http://www.ncbi.nlm.nih.gov/pubmed/17629472?tool=bestpractice.com
However, other studies have found that influenza virus is the most common cause of CAP in adults.[7]Alimi Y, Lim WS, Lansbury L, et al. Systematic review of respiratory viral pathogens identified in adults with community-acquired pneumonia in Europe. J Clin Virol. 2017 Oct;95:26-35.
http://www.ncbi.nlm.nih.gov/pubmed/28837859?tool=bestpractice.com
[18]Jain S, Self WH, Wunderink RG, et al. Community-acquired pneumonia requiring hospitalization among US adults. N Engl J Med. 2015 Jul 30;373(5):415-27.
http://www.nejm.org/doi/full/10.1056/NEJMoa1500245
http://www.ncbi.nlm.nih.gov/pubmed/26172429?tool=bestpractice.com
In Europe and the US, S pneumoniae accounts for about 30% to 35% of cases.[14]Cillóniz C, Ewig S, Polverino E, et al. Microbial aetiology of community-acquired pneumonia and its relation to severity. Thorax. 2011 Apr;66(4):340-6.
http://thorax.bmj.com/content/66/4/340.long
http://www.ncbi.nlm.nih.gov/pubmed/21257985?tool=bestpractice.com
[19]Almirall J, Bolíbar I, Vidal J, et al. Epidemiology of community-acquired pneumonia in adults: a population-based study. Eur Respir J. 2000 Apr;15(4):757-63.
http://erj.ersjournals.com/content/15/4/757.long
http://www.ncbi.nlm.nih.gov/pubmed/10780770?tool=bestpractice.com
Other bacterial causes include Haemophilus influenzae, Staphylococcus aureus (including MRSA), group A streptococci, and Legionella spp. For example, Legionella pneumophila (especially serogroup 1) accounts for 2% to 6% of CAP in immunocompetent patients.[20]Mandell LA, Marrie TJ, Grossman RF, et al; The Canadian Community-Acquired Pneumonia Working Group. Canadian guidelines for the initial management of community-acquired pneumonia: an evidence-based update by the Canadian Infectious Diseases Society and the Canadian Thoracic Society. Clin Infect Dis. 2000 Aug;31(2):383-421.
http://cid.oxfordjournals.org/content/31/2/383.full
http://www.ncbi.nlm.nih.gov/pubmed/10987698?tool=bestpractice.com
Atypical bacteria are also common causes, although they vary in frequency depending on the year and any epidemics.[17]Almirall J, Boixeda R, Bolíbar I, et al. Differences in the etiology of community-acquired pneumonia according to site of care: a population-based study. Respir Med. 2007 Oct;101(10):2168-75.
http://www.resmedjournal.com/article/S0954-6111%2807%2900194-1/fulltext
http://www.ncbi.nlm.nih.gov/pubmed/17629472?tool=bestpractice.com
[21]Marchello C, Dale AP, Thai TN, et al. Prevalence of atypical pathogens in patients with cough and community-acquired pneumonia: a meta-analysis. Ann Fam Med. 2016 Nov;14(6):552-66.
http://www.annfammed.org/content/14/6/552.long
http://www.ncbi.nlm.nih.gov/pubmed/28376442?tool=bestpractice.com
The incidence of atypical pathogens in community-acquired pneumonia is approximately 22% globally, but this varies with location.[22]Arnold FW, Summersgill JT, Ramirez JA. Role of atypical pathogens in the etiology of community-acquired pneumonia. Semin Respir Crit Care Med. 2016 Dec;37(6):819-28.
http://www.ncbi.nlm.nih.gov/pubmed/27960206?tool=bestpractice.com
Commonly reported atypical bacteria include Mycoplasma pneumoniae, Chlamydophila pneumoniae, Chlamydia psittaci and Coxiella burnetii.[1]Lim WS, Baudouin SV, George RC, et al; Pneumonia Guidelines Committee of the BTS Standards of Care Committee. British Thoracic Society guidelines for the management of community acquired pneumonia in adults: update 2009. Thorax. 2009 Oct;64(suppl 3):iii1-55.
https://thorax.bmj.com/content/64/Suppl_3/iii1.long
http://www.ncbi.nlm.nih.gov/pubmed/19783532?tool=bestpractice.com
These pathogens are difficult to diagnose early in the illness and are sensitive to antibiotics other than beta-lactams (e.g., macrolides, tetracyclines or fluoroquinolones).[1]Lim WS, Baudouin SV, George RC, et al; Pneumonia Guidelines Committee of the BTS Standards of Care Committee. British Thoracic Society guidelines for the management of community acquired pneumonia in adults: update 2009. Thorax. 2009 Oct;64(suppl 3):iii1-55.
https://thorax.bmj.com/content/64/Suppl_3/iii1.long
http://www.ncbi.nlm.nih.gov/pubmed/19783532?tool=bestpractice.com
M pneumoniae accounts for up to 37% of CAP patients treated as outpatients and 10% of patients who are hospitalised.[14]Cillóniz C, Ewig S, Polverino E, et al. Microbial aetiology of community-acquired pneumonia and its relation to severity. Thorax. 2011 Apr;66(4):340-6.
http://thorax.bmj.com/content/66/4/340.long
http://www.ncbi.nlm.nih.gov/pubmed/21257985?tool=bestpractice.com
C pneumoniae accounts for 5% to 15% of cases of CAP. However, a Dutch study identified C psittaci by polymerase chain reaction (PCR) of sputum (when available) as a cause of CAP in 4.8% of cases.[23]Spoorenberg SM, Bos WJ, van Hannen EJ, et al. Chlamydia psittaci: a relevant cause of community-acquired pneumonia in two Dutch hospitals. Neth J Med. 2016 Feb;74(2):75-81.
http://www.ncbi.nlm.nih.gov/pubmed/26951352?tool=bestpractice.com
One German study identified C burnetii by PCR and/or antibody detection as the cause of CAP in 3.5% of patients.[24]Schack M, Sachse S, Rödel J, et al. Coxiella burnetii (Q fever) as a cause of community-acquired pneumonia during the warm season in Germany. Epidemiol Infect. 2014 Sep;142(9):1905-10.
https://www.cambridge.org/core/journals/epidemiology-and-infection/article/coxiella-burnetii-q-fever-as-a-cause-of-communityacquired-pneumonia-during-the-warm-season-in-germany/479C3CA472D5CF8C19C3D8CED134DC6D
http://www.ncbi.nlm.nih.gov/pubmed/24252152?tool=bestpractice.com
Pseudomonas aeruginosa may also be prevalent in patients with pneumonia, depending on the region; however, it is more common in hospital-acquired and ventilator-associated pneumonia compared with CAP. It accounted for 7.7% of all isolates in CAP in a systematic review in China.[25]Ding C, Yang Z, Wang J, et al. Prevalence of Pseudomonas aeruginosa and antimicrobial-resistant Pseudomonas aeruginosa in patients with pneumonia in mainland China: a systematic review and meta-analysis. Int J Infect Dis. 2016 Aug;49:119-28.
http://www.sciencedirect.com/science/article/pii/S1201971216310992
http://www.ncbi.nlm.nih.gov/pubmed/27329135?tool=bestpractice.com
Respiratory viruses are reported in about 10% to 30% of immunocompetent adults hospitalised with CAP.[14]Cillóniz C, Ewig S, Polverino E, et al. Microbial aetiology of community-acquired pneumonia and its relation to severity. Thorax. 2011 Apr;66(4):340-6.
http://thorax.bmj.com/content/66/4/340.long
http://www.ncbi.nlm.nih.gov/pubmed/21257985?tool=bestpractice.com
[26]Jokinen C, Heiskanen L, Juvonen H, et al. Microbial etiology of community-acquired pneumonia in the adult population of 4 municipalities in eastern Finland. Clin Infect Dis. 2001 Apr 15;32(8):1141-54.
http://cid.oxfordjournals.org/content/32/8/1141.long
http://www.ncbi.nlm.nih.gov/pubmed/11283803?tool=bestpractice.com
[27]Jennings LC, Anderson TP, Beynon KA, et al. Incidence and characteristics of viral community-acquired pneumonia in adults. Thorax. 2008 Jan;63(1):42-8.
http://thorax.bmj.com/content/63/1/42.long
http://www.ncbi.nlm.nih.gov/pubmed/17573440?tool=bestpractice.com
[28]Burk M, El-Kersh K, Saad M, et al. Viral infection in community-acquired pneumonia: a systematic review and meta-analysis. Eur Respir Rev. 2016 Jun;25(140):178-88.
http://err.ersjournals.com/content/25/140/178.long
http://www.ncbi.nlm.nih.gov/pubmed/27246595?tool=bestpractice.com
Influenza virus A/B, respiratory syncytial virus, adenovirus, rhinovirus, and parainfluenza virus are the most common viral causes of CAP in immunocompetent adults. Newer pathogens reported to cause CAP include metapneumovirus and coronaviruses.[29]Wunderink RG, Waterer G. Advances in the causes and management of community acquired pneumonia in adults. BMJ. 2017 Jul 10;358:j2471.
http://www.ncbi.nlm.nih.gov/pubmed/28694251?tool=bestpractice.com
Detection of viral causes is increasing because of the use of PCR.
Polymicrobial aetiology in CAP varies from 5.7% to 13%, depending on the population and the microbiological diagnostic test used.[14]Cillóniz C, Ewig S, Polverino E, et al. Microbial aetiology of community-acquired pneumonia and its relation to severity. Thorax. 2011 Apr;66(4):340-6.
http://thorax.bmj.com/content/66/4/340.long
http://www.ncbi.nlm.nih.gov/pubmed/21257985?tool=bestpractice.com
[27]Jennings LC, Anderson TP, Beynon KA, et al. Incidence and characteristics of viral community-acquired pneumonia in adults. Thorax. 2008 Jan;63(1):42-8.
http://thorax.bmj.com/content/63/1/42.long
http://www.ncbi.nlm.nih.gov/pubmed/17573440?tool=bestpractice.com
[30]Johansson N, Kalin M, Tiveljung-Lindell A, et al. Etiology of community-acquired pneumonia: increased microbiological yield with new diagnostic methods. Clin Infect Dis. 2010 Jan 15;50(2):202-9.
http://cid.oxfordjournals.org/content/50/2/202.long
http://www.ncbi.nlm.nih.gov/pubmed/20014950?tool=bestpractice.com